Pictures: Green Walls May Cut Pollution in Cities

1 / 8

These vegetated surfaces don't just look pretty. They have other benefits as well, including cooling city blocks, reducing loud noises, and improving a building's energy efficiency. (See a complete list of green wall benefits.)

What's more, a recent modeling study shows that green walls can potentially reduce large amounts of air pollution in what's called a "street canyon," or the corridor between tall buildings.

For the study, Thomas Pugh, a biogeochemist at the Karlsruhe Institute of Technology in Germany, and his colleagues created a computer model of a green wall with generic vegetation in a Western European city. Then they recorded chemical reactions based on a variety of factors, such as wind speed and building placement.

Compared with reducing emissions from cars, little attention has been focused on how to trap or take up more of the pollutants, added Pugh, whose study was published last year in the journal Environmental Science & Technology.

That's why the green-wall study is "putting forward an alternative solution that might allow [governments] to improve air quality in these problem hot spots," he said.

—Christine Dell'Amore

Photograph by Bertrand Garbel, Hemis.fr/Getty Images

Vertical Garden

Living walls can be indoors or outdoors, such as this one blooming on the Citi Data Centre in Frankfurt, Germany.

There are three types of green walls: Living walls, green facades, and retaining living walls, he said.

In green facades, vines and climbing plants or cascading ground covers grow into supporting structures, according to the Green Roofs for Healthy Cities website.

Living walls are composed of pre-vegetated panels, modules, planted blankets, or bags that are affixed to a structural wall or free-standing frame. Finally, retaining living walls are engineered living structures designed to stabilize a slope.

Green walls are particularly useful in reducing pollution in street canyons, which are "a particular kind of oddity in that they tend to trap air within them, while over the city as a whole the air circulates freely," Thomas Pugh noted. (Take a pollution quiz.)

Because a green wall's vegetation is working with a much smaller volume of air in a street canyon, he added, it's more effective at absorbing or trapping the pollutants than if it was tackling a whole mass of air above the city.

However, Pugh cautioned, "we need more research to understand just how effective green walls can really be in real locations."

The ability of green walls, such as this one at the Musée du Quai Branly in Paris, to cool street canyons is especially key in the summertime, Thomas Pugh noted.

"If you have a surface covered in plants," he said, "when the sun's heat hits them, instead of just warming up they send a lot of energy back into the atmosphere as water vapor. Whereas a concrete wall would heat up and send [the heat] back into the canyon." (Learn more about Earth's atmosphere.)

Many living wall systems also have built-in irrigation systems in which water is harvested at the bottom of the wall and recirculated back to the top, as seen above.

The types of plants on green walls differ, but can include grasses, vines, and other climbing plants—all of which must have strong root structures, Green Roofs for Healthy Cities' Paul Erlichman said. (Test your environmental knowledge.)